“About 250 flights have already been canceled, mostly over Scotland” Kyla Evans, a spokeswoman for Eurocontrol, the Brussels-based agency that coordinates air traffic management across the region, told the newspaper. “We would expect up to 500 flights to be canceled today. But it would very much depend on how the ash cloud moves.”

In London, the Met Office’s Volcanic Ash Advisory Center said there was a strong possibility that the ash cloud may affect parts of Denmark and southern Scandinavia. The center also warned that low pressure systems expected across northern Britain this week mean “wind direction is likely to be quite variable,” making forecasting the progress of the cloud difficult.

The eruption led President Obama, in Europe on a six-day visit that will include a Group of Eight meeting in France, to shorten a visit to Ireland on Monday and fly to London a day early.

Airlines urged travelers, particularly those headed to Scotland or Iceland, to verify the status of their flights online before leaving for the airport.